This invention relates to electro-optic systems, and more specifically, to an electro-optic system wherein a dielectric anisotropic material in its optically uniaxial state is used. Furthermore, this invention includes electro-optic cells and imaging systems embodying the inventive dielectric anisotropic electro-optic system.
Dielectric anisotropic materials include some of the liquid crystalline materials, and non-liquid crystalline materials such as anisotropic particles in carrier fluids.
Recently, there has been substantial interest in the discovery of more useful applications for the class of substances known as "liquid crystals." The name liquid crystals has become generic to liquid crystalline materials which exhibit dual physical characteristics, some of which are typically associated with liquids and others which are typically unique to solids. Liquid crystals exhibit mechanical characteristics, such as viscosities, which are normally associated with liquids. The optical characteristics of liquid crystals are more similar to those characteristics ordinarily unique to crystalline solids. In liquids or fluids, the molecules are typically randomly distributed and oriented throughout the mass of the material. Conversely, in crystalline solids the molecules are generally rigidly oriented and arranged in a specific crystalline structure. Liquid cyrstals resemble solid crystals in that the molecules of the liquid crystalline compositions are regularly oriented in a fashion analogous to, but less extensive than, the molecular orientation and structure in a crystalline solid. Many substances have been found to exhibit liquid crystalline characteristics in a relatively narrow temperature range; below the temperature range the substance typically appear as crystalline solids, and above that temperature range they typically appear as liquids. Liquid crystals are known to appear in three different mesomorphic forms; the smectic, the nematic and cholesteric. In each of these structures, the molecules are typically arranged in a unique orientation. In the nematic liquid crystalline mesophase structure, the major axes of the molecules lie approximately parallel to each other, but the molecules are typically not specifically organized in any other fashion.
Nematic liquid crystals are known to be responsive to electrical fields, and have been used in various electro-optic cells and imaging systems, for example as disclosed in Williams U.S. Pat. No. 3,322,485, Freund et al., U.S. Pat. 3,364,433; Heilmeier et al., U.S. Pat. No. 3,499,112; and Goldmacher et al., U.S. Pat. No. 3,499,702. Most of the known nematic liquid crystalline light valves and display devices make use of the dynamic light scattering characteristics of layers of nematic liquid crystalline material which have electrical fields placed across the thickness of the layer. The dynamic light scattering is believed to be due to the differential alignment of domains or swarms of birefringent liquid crystalline molecules in the electric field affected areas in such systems.
In the smectic structure the molecules are arranged in layers with their major axes approximately parallel to each other and approximately normal to the planes of said layers. Within a given layer the molecules may be organized in uniform rows, or randomly distributed throughout the layer, but in either case the major axes are still approximately normal to the plane of the layer. The attractive forces between layers are relatively weak so that the layers are free to move in relation to each other, thereby providing the smectic liquid crystalline substance with the mechanical properties of a planar or two-dimensional, soap-like fluid.
In the cholesteric structure, the molecules are believed to be arranged in definite layers as in the smectic structure; however, within a given layer, the molecules are believed to be arranged with their major axes approximately parallel in a fashion resembling the structure of nematic liquid crystals. Because the major axes of the molecules in the cholesteric structure are believed to be parallel to the planes of the layers, the molecular layers are very thin. The cholesteric structure derives its name from the fact that materials exhibiting the cholesteric liquid crystalline structure typically have molecules which are derivatives of cholesterol or which are shaped very similarly to molecules of cholesterol. Because of the shape of the cholesteric molecule, in the cholesteric structure the direction of the major axes of the molecules in each of the aforementioned thin layers is displaced slightly from the direction of the major molecular axes in the adjacent molecular layers. When compared to a hypothetical straight line axis passing through a cholesteric liquid crystalline substance and perpendicular to the molecular planes within said substance, the angular displacement of the direction of the molecular axes within each adjacent molecular layer traces out a helical path around the hypothetical straight line axis.
Cholesteric liquid crystals are known to be responsive to electrical fields (see Harper, W. J. "Voltage Effects in Cholesteric Liquid Crystals," in Molecular Crystals, Vol. 1, 1966, pages 325-332). The effects of an electrical field upon a sample of a liquid crystalline substance has typically been observed in a cell comprising a film of liquid crystals sandwiched between transparent electrodes, as discloed, for example in copending application Ser. No. 646,532, filed June 16, 1967, now U.S. Pat. No. 3,804,618 and French Pat. No. 1,484,584. In both of these references liquid crystals are used for imaging in response to electrical fields. The imaging in prior art devices has typically comprised modification of the optical properties of the liquid crystalline substance maintaining its original liquid crystalline mesophase form, i.e., smectic, nematic, or cholesteric. Recently, however, NMR spectral studies have shown that a magnetic field may cause a cholesteric liquid crystalline substance to go through a phase transition to the nematic liquid crystalline structure (see Sackmann, Meiboom, and Snyder, "On the Relation of Nematic to Cholesteric Mesophases," in J. Am. Chem. Soc., 89:73, Nov. 8, 1967). Also, U.S. Pat. No. 3,652,148, Wysocki et al, discloses the application of an electrical field to transform a cholesteric liquid crystal to a nematic liquid crystalline structure.
Recently, Haas et al., U.S. Pat. No. 3,687,515 disclosed an electroc-optic system wherein a layer of spontaneously homeotropic textured optically uniaxial nematic liquid crystalline composition with the optic axis normal to the plane of the layer was rendered optically biaxial by the application of an electrical field perpendicular to the uniaxial optic axis. When the field is removed, the composition naturally relaxes back into its optically uniaxial, homeotropic texture.
In new and growing areas of imaging technology, new methods, apparatus, compositions, and articles of manufacture are often discovered for the application of the new imaging technology in a new mode. The present invention relates to a new and advantageous system for imaging an electro-optic cell containing a dielectric anisotropic material in its optically uniaxial state.